1. Field of the Invention
The present invention relates to an optical shutter device, and more particularly to an optical shutter device which has polarization shutter chips made of PLZT and is used to write an image on a receptor surface such as a photosensitive member or the like.
2. Description of Related Art
In order to write an image (latent image) on a photographic paper using a silver halide photosensitive material and an electrophotographic photosensitive member, there have been heretofore provided various solid scanning type optical writing apparatuses for controlling turning-on and turning-off of light on a pixel-by-pixel basis by the use of polarization shutter chips made of PLZT having an electro-optic effect of a high Kerr constant. It is effective to adapt such polarization shutter chips to have a stereostructure in order to achieve a high contrast and a low driving voltage. A multi-chip system is also employed in which a plurality of PLZT chips with plural pixels are linearly aligned in order to constitute a longer one-dimensional optical shutter module. However, thus constituted one-dimensional optical shutter module has a problem that light leaks from vertical electrodes, pixel separating grooves and chip cut surfaces (seams), which may lower the contrast.
The leakage of light is described. FIG. 19 shows an example of a conventional PLZT shutter chip having a stereostructure. This shutter chip 100 is provided with a common electrode 102 and separate electrodes 103 extending to the vertical surfaces of transmittable portions 101. Each transmittable portion 101 corresponding to one pixel rotates the plane of polarization of light when an electric field is generated by the application of a voltage to the electrodes 102 and 103, and thereby, the light is turned on and off pixel by pixel. The electrodes 102 and 103 are formed on vertical surfaces parallel to an optical path, so that the driving voltage (half-wavelength voltage) capable of obtaining a maximum quantity of transmitted light is reduced.
Grooves 104 for separating the transmittable portions 101 and the separate electrodes 103 from one another enable each of the transmittable portions 101 to be independently driven. The grooves 104 also prevent an operation of the transmittable portion 101 due to the application of an electric field from the adjacent transmittable portions 101, that is, a crosstalk.
.theta. is assumed to be taken as the angle of the direction of polarization of incident linearly polarized light to the direction of electric field generated by the electrodes 102 and 103. In this case, when the half-wavelength voltage is applied to the electrodes 102 and 103, the relationship between the intensity I.sub.0 of incident light and the intensity I of emergent light is represented by the following expression (1). EQU I.varies.I.sub.0 sin.sup.2 2.theta. (1)
.theta. is then set to 45.degree. in order to obtain the maximum quantity of transmitted light.
The optical shutter module is constituted so that a plurality of the chips 100, each having a plurality of the transmittable portions 101, are linearly aligned on a glass substrate or, if not transparent, a substrate having an opening formed thereon, on which a polarizer and an analyzer are arranged on the incident side and the emergent side, respectively.
FIG. 20 shows the optical shutter module constituted as described above. The transmittable portions 101 are arranged in two rows in a zigzag pattern so that an optical signal may be provided to a receptor surface to be scanned without any gap in a main scanning direction. Therefore, the grooves 104 for separating the transmittable portions 101 and the separate electrodes 103 are formed to satisfy geometrical conditions for scanning without any gap regardless of the direction of electric field. In FIG. 20, the separating grooves 104 are at an angle of 11.degree. to the direction of electric field. Since the multi-chip system is employed, seams 110 among the chips 100 are parallel to the separating grooves 104.
In such an optical shutter module, space areas such as the separating grooves 104 and the seams 110, in addition to both sides of the transmittable portions 101, are therefore formed. Light reflected from the vertical surfaces in these space areas results in so-called leakage light passing through the analyzer whose direction of polarization is perpendicular to the direction of polarization of incident linearly polarized light, so that the contrast of the image is thus lowered.
The factors are described with reference to FIG. 21. Part of light which is incident to the module at a certain angle is incident to the vertical surfaces 104a and 110a from the air and then reflected from the surfaces 104a and 110a, and this light is called externally reflected light and denoted by La. The light La is composed of a component of p-polarized light and a component of s-polarized light having reflectance properties depending on the incident angle as shown by curves Rp and Rs in FIG. 22, respectively. The p-polarized light means the polarized light parallel to the reflecting surface, while the s-polarized light means the polarized light perpendicular to the reflecting surface. The refractive index of PLZT is 2.5. As can be seen from FIG. 22, the respective reflectance properties Rp and Rs of the p-polarized light and the s-polarized light of the externally reflected light greatly differ. Thus, the incident linearly polarized light outgoes in the form of polarized light whose direction of polarization is changed, passes through the analyzer and then results in leakage light.
On the other hand, in FIG. 21, light (internally reflected light) Lb is incident to the vertical surfaces 104a and 110a from the inside of the chip 100 and then reflected from the surfaces 104a and 110a. The light Lb is composed of a component of p-polarized light and a component of s-polarized light having phase-angle properties depending on the incident angle as shown by curves .delta.p and .delta.s in FIG. 23, respectively. As can be seen from FIG. 23, the respective phase-angle properties .delta.p and .delta.s of the p-polarized light and the s-polarized light of the internally reflected light greatly differ. Thus, the incident linearly polarized light is changed into elliptically polarized light, passes through the analyzer and then results in leakage light.
In short, it has been unavoidable that leakage light is caused due to the externally and internally reflected light. Even when the driving voltage is turned off, light outgoes from the optical shutter module, which thus causes the contrast to be lowered on the receptor surface.